In February, we released the first preview of HTTP Strict Transport Security in Internet Explorer 11 in the Windows 10 Insider Preview. The HTTP Strict Transport Security (HSTS) policy protects against variants of man-in-the-middle attacks that can strip TLS out of communications with a server, leaving the user vulnerable.

With today’s monthly security updates (KB 3058515), we’re bringing the protections offered by HSTS to Internet Explorer 11 on Windows 8.1 and Windows 7. HSTS is also available in both Internet Explorer 11 and Microsoft Edge on Windows 10.

Site developers can use HSTS policies to secure connections by opting in to an HSTS preload list, which registers websites to be hardcoded by Microsoft Edge, Internet Explorer, and other browsers to redirect HTTP traffic to HTTPS. Communications with these websites from the initial connection are automatically upgraded to be secure. Like other browsers which have implemented this feature, Microsoft Edge and Internet Explorer 11 base their preload list on the Chromium HSTS preload list.

Alternatively, sites not on the preload list can enable HSTS via the Strict-Transport-Security HTTP header. After an initial HTTPS connection from the client containing the HSTS header, any subsequent HTTP connections are redirected by the browser to be secured via HTTPS.

When we initially announced HSTS in Windows 10, we noted that mixed content is not supported on servers supporting HSTS. With today’s updates, this is still the case in Microsoft Edge on Windows 10 – mixed content is always blocked on these servers. For Internet Explorer 11 on Windows 10, Windows 8.1, and Windows 7, the Information bar will prompt the user to proceed in mixed content scenarios.

In addition to the HSTS updates, this month’s Internet Explorer updates include 24 security fixes, which you can see detailed on TechNet for more details.